Integrated magnetoresistive sensor

ABSTRACT

A magnetoresistor is monolithically integrated with an active circuit by growing a thin film magnetoresistor on a semiconductor substrate after the substrate has been doped and annealed for the active devices. The magnetoresistor is grown through a window in a mask, with the mask and magnetoresistor materials selected such that the magnetoresistor is substantially non-adherent to the mask. InSb is preferred for the magnetoresistor, Si 3  N 4  for the mask and GaAs for the substrate. The non-adherence allows the mask to be substantially thinner than the magnetoresistor without impairing the removal of the mask after the magnetoresistor has been established.

This is a division of application Ser. No. 08/161,021 filed Dec. 3,1993.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to magnetoresistive sensors, and moreparticularly to methods for fabricating integrated magnetoresistivesensors and the resulting sensor structure.

2. Description of the Related Art

Varying magnetic fields have been used in the past as a sensingmechanism for,moving parts, such as rotating elements in an automobile.The rotating element causes a magnetic field to periodically vary, andthe rate of variation is sensed as an indication of the rotational speedand/or position.

A simplified system for measuring the rotational speed of a rotatingelement is shown in FIG. 1. A wheel or gear 2 has a plurality of spacedprotruding teeth 4 and rotates below a magnet 6. A magnetoresistor 8that is provided on its own substrate is positioned in the magneticfield 10 between the magnet and wheel, while amplification and (ifdesired) digitizing circuits are provided on one or more separatesubstrates 12 that are electrically connected to the magnetoresistor 8.

The rotating wheel 2 is formed from a magnetic material, and thusattracts the field from magnet 6. The magnetic field at themagnetoresistor 8 is strongest when one of the teeth 4 is locateddirectly below magnet 6, thus minimizing the distance between the magnetand the wheel. As the wheel rotates the field tends to bend along withthe movement of the tooth, and thus traverses a greater distance as thetooth moves away from the magnet. These effects produce a reduction inthe magnetic field strength at the magnetoresistor 8, which reaches aminimum when the magnet 6 is midway between two teeth 4. The magneticfield strength at the magnetoresistor increases again as the next toothapproaches, reaching a maximum when that tooth is located directly belowthe magnet. The field strength at the magnetoresistor thus variesperiodically as the wheel continues to rotate, causing the resistance ofmagnetoresistor 2 to fluctuate in a similar fashion. This results in aperiodically varying output from the amplifier circuitry 12.

It would be less expensive and reduce the bulk and complexity of theapparatus if the magnetoresistor could be integrated with the outputcircuitry in a single monolithic structure. However, this has not bepractical in the past. Magnetoresistors have commonly been grown incrystalline form, and then cleaved into individual resistor elementsthat are simply glued to respective substrates. Other magnetoresistorfabrication techniques are known in which the magnetoresistive elementis monolithically grown on a substrate. For example, in U.S. Pat. No.3,898,359 to Nadkarni, a thin film layer of antimony or arsenic isapplied over a layer of indium on an insulating substrate that has beencoated with chromium or nickel. The antimony or arsenic film is themchemically combined with the indium to form an InSb or InAsmagnetoresistor. A series of transverse indium Hall effect shortingstrips are formed on the upper surface of the magnetoresistor to shortcircuit the Hall fields that would otherwise be built up. In anothermonolithic magnetoresistor construction, an InSb magnetoresistorresistor is epitaxially grown on a GaAs substrate for use in infraredfocal plane arrays; Chiang and Bedair, "Growth of InSb and InAs_(1-x)Sb_(x) by OM-CVD", J, Electrochem. Society; Solid-State Science andTechnology, October 1984, pages 2422-2426. As with U.S. Pat. No.3,898,359, this article discloses the fabrication of a magnetoresistorby itself, without any associated circuitry on the same substrate.

In fact, although GaAs is particularly suited as a substrate for InSbbecause it has thermal expansion characteristics that are very close tothose of InSb, InSb is not compatible with the annealing of dopants inGaAs. This is because the annealing is typically performed at about 850°C., while InSb cannot withstand temperatures greater than about 450° C.This is a significant limitation in the achievement of a monolithicallyintegrated magnetoresistor and processing circuit, since InSb is themost magnetosensitive material currently known. It would also not befeasible to grow an InSb magnetoresistor on the same substrate with anintegrated circuit that has already been fabricated, since InSb istypically grown at about 400° C., while the ohmic contacts of anintegrated circuit begin to degrade at temperatures above about 250° C.

Even if the temperature problems could be overcome, there are otherserious obstacles to the monolithic integration of a magnetoresistorwith associated output circuitry. To grow a magnetoresistor at a desiredlocation on the substrate, the usual approach would be to provide agrowth mask over the entire substrate, with a window at the desiredlocation for the magnetoresistor. InSb would then be grown over the maskand onto the substrate through the window, followed by removing the maskand the overlying InSb to leave the magnetoresistor material only in thewindow area. However, an InSb magnetoresistor is typically about anorder of magnitude thicker than a typical pattern mask, and a continuouslayer of the InSb over the entire mask area would prevent the solventfrom reaching and removing the mask and leaving a clean substratesurface. It would also not be feasible to grow InSb over the entiresubstrate and then etch away the unwanted areas, since the etchant wouldalso attack the underlying GaAs.

A magnetosensitive device that is monolithically integrated withassociated output circuitry is described in Lepkowski et al., "A GaAsIntegrated Hall Sensor/Amplifier", IEEE Electron Device Letters, Vol.Edl-7, No 4, April 1986, pages 222-224. This device, however, uses aHall sensor rather than a magnetoresistor. The Hall sensor is formedfrom the GaAs substrate material itself, rather than from a separatemagnetoresistor material such as InSb, and therefore does not face thetemperature and growth obstacles described above. Although they areeasier to integrate with output circuitry, Hall sensors are not asmagnetically sensitive as magnetoresistors.

SUMMARY OF THE INVENTION

The present invention seeks to provide a method for fabricating asensitive magnetoresistor such as InSb in a monolithic integratedstructure along with output circuitry, without degrading either themagnetoresistor or the circuitry, and thereby achieving a substantiallylower cost than prior magnetoresistive sensors without sacrificingperformance.

To achieve these goals, a semiconductor substrate (preferably GaAs) isdoped in the areas that are intended for active devices of themagnetoresistive output circuitry, and then subjected to a hightemperature anneal. A mask is next formed on the substrate with a windowat a desired magnetoresistor location, and a thin film magnetoresistoris grown within the window. The mask and magnetoresistor materials areselected so that the magnetoresistor does not adhere to the mask;preferred materials are Si₃ N₄ for the mask and InSb for themagnetoresistor. This allows the magnetoresistor be grown to asubstantially greater thickness than the mask, without interfering withthe removal of the mask after the magnetoresistor has been grown. Themask is removed after an additional optional patterning of themagnetoresistor, and conductive contacts are then made to the dopedareas to establish the active devices, and to interconnect themagnetoresistor with the active devices. Although a small residue ofmagnetoresistor material may form on the mask, it will not be enough toprevent a clean removal of the mask.

The magnetoresistor can be monolithically integrated with an outputamplifier and digitizing circuit to produce a TTL (transistor-transistorlogic) compatible output signal. A considerable improvement insignal-to-noise ratio is achievable, making the sensor more applicableto environments in which the signal generation is considered to bemarginal for reliable operation. In addition, temperature compensationcan be achieved by integrating a pair of resistors in close proximity onthe chip, with one of the resistors magneto sensitive and the otherrelatively insensitive.

These and other features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription, taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of a known rotational speed sensor,described above, that uses a magnetoresistor;

FIGS. 2a-2g are simplified and not-to-scale sectional views thatillustrate sequential steps in the fabrication of a monolithicallyintegrated magnetoresistor and output circuit in accordance with theinvention;

FIG. 3 is a plan view of a portion of a magnetoresistor and

FIG. 4 is a schematic diagram of an amplifier/digitizing/temperaturecompensation circuit that can be monolithically integrated with amagnetoresistor in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

One or more magnetoresistors are monolithically integrated with othercircuitry on a common IC chip with a unique fabrication process, withthe initial steps of the preferred embodiment illustrated in FIG. 2a.The structure is fabricated on a semi-insulating semiconductor substrate14, which is preferably GaAs for the preferred InSb magnetoresistor.Other semiconductor substrate materials such as silicon and IndiumPhosphide might also be used, although silicon and InSb have differentthermal coefficients of expansion that can make their combinationdifficult, and InSb is a relatively more primitive state of developmentas a substrate material. Other magnetoresistive materials such as InAscan also be used, although InAs has a considerably lowermagnetoresistivity than InSb. An InSb magnetoresistor on a GaAssubstrate will be assumed hereinafter.

Areas of the substrate 14 that are intended for active electricaldevices, indicated by numerals 16 and 18, are first doped to the desiredconductivity. The doping is preferably performed by ion implantation,indicated by arrows 20, rather than by a higher temperature gaseousdiffusion process. Areas of the substrate for which doping is notdesired, such as the area to be occupied by the magnetoresistor, areprotected from the implantation by a photoresist 22 or other suitablemask.

The wafer is next coated with a layer of silicon nitride (Si₃ N₄) 24, asillustrated in FIG. 2b, and then annealed at a temperature of about850°-890° C. to active the dopant in the GaAs. The silicon nitride layer24 is grown at a conventional pressure of about 650 m Torr andtemperature of about 250° -350° C., and coats the entire wafer,generally to a thickness of only about 0.2 microns. It is then patternedwith a suitable mask (not shown) over the doped areas 16 and 18, with anopening formed in the mask over the undoped area between the two dopedareas where the magnetoresistor is to be formed. The unmasked siliconnitride layer is then removed, preferably with a dilute hydrofluoricacid etchant, to open a window 26 (shown in FIG. 2c) in the siliconnitride for the later formation of the magnetoresistor. The patterningmask over the remaining silicon nitride layer is then removed.

In the next step, illustrated in FIG. 2d, an InSb crystal 28 is grown inthe silicon nitride window, preferably by metal organic chemical vapordeposition (MOCVD) at a temperature of about 380° -400° C., oralternately by molecular beam epitaxy (MBE), as indicated by arrows 30.The magnetoresistor material is grown within the window to a preferredthickness of about 2 microns. However, if the growth pressure is kept tonot significantly more than 1 atmosphere, the InSb will not adhere tothe surface of the silicon nitride layer 24. At most, only a smallresidue 32 of InSb will grow on the silicon nitride.

The choice of a mask material to which the magnetoresistive materialwill not adhere is an important aspect of the invention. The siliconnitride layer is normally only about 0.2 microns thick, as compared tothe typical InSb thin film thickness of about 2 microns. Thus, if acontinuous and substantially thicker InSb film were formed both in thewindow 26 and over the adjacent silicon nitride layer, the InSb would ineffect seal the silicon nitride and inhibit access by a solvent that isused at a later stage in the fabrication process to remove the mask.

After the magnetoresistor 28 has been formed, it may be furtherpatterned by coating the substrate with an etch mask 34 (illustrated inFIG. 2e) except where magnetoresistor removal is desired, and thenetching away the unmasked portion. A combination of lactic and nitricacid is preferred for etching InSb. This etchant also attacks GaAs,although at a rate about 5-7 times slower than InSb. However, it is nota problem if the etchant degrades the GaAs surface within the windowarea, since this portion of the substrate is not used for any activedevices.

After any optional patterning of the magnetoresistor has been completedand the patterning mask 34 removed, the silicon nitride layer 24 overthe doped substrate areas is removed, preferably with the same dilutehydrofluoric acid etch as mentioned above. Any small residue of InSbthat may have formed on the silicon nitride will not be enough tointerfere with the etch, and is simply washed away along with thesilicon nitride. The resulting magnetoresistor 28 that has beenmonolithically integrated with the doped GaAs areas is illustrated inFIG. 2f.

Ohmic contacts to the GaAs, such as for source and drain contacts of afield effect transistor (FET), are then established. During this stepwith the magnetoresistor 28 and the remainder of the substrate areprotected by a photoresist layer that is lifted off along with theoverlying metal when the contacts have been established. A photoresistpattern is next laid down for the deposition of gate metal, along withcontacts and shorting strips for the magnetoresistor; a preferred metalfor this purpose is TiPtAu. A second metallization layer is thenpatterned to interconnect the contacts established with the firstmetallization layer, and additional metallizations may be built up ifdesired in a conventional manner.

A completed magnetoresistor and pair of FETs that have been fabricatedas a monolithic integrated structure in this fashion are illustrated inFIG. 2g. The FETs 36 and 38 include source contacts 36s and 38s, draincontacts 36d and 38d, and gate contacts 36g and 38g. The magnetoresistor28 is shown connected to the transistor drains by metallizedinterconnects 40 and 42; for purposes of simplification the FET sourceand gate connections are not shown, nor is a passivation layer ofsilicon nitride that would normally coat the entire circuit. While FETsof only one conductivity are shown, complementary FETs of oppositeconductivity could also be formed by an appropriate doping of thesubstrate prior to growing the magnetoresistor. For example, if FETs 36and 38 are n-channel devices, p-channel FETs could also be fabricated byproviding p-wells.

A section of the magnetoresistor 28 is illustrated in FIG. 3, includingthe Hall effect shorting strips 44 that are formed in the samemetallization step as the magnetoresistor contacts 40 and 42 in FIG. 2g.The magnetoresistor may have any desired shape; a serpentine shape iscommonly used to minimize area requirements on the substrate.

FIG. 4 is a schematic diagram of a monolithic integratedmagnetosensitive circuit that can be implemented with the invention. Thecircuit per se is known, and is described (except for its outputdigitizing section) in U.S. Pat. No. 5,308,130 to Eck et al., assignedto Hughes Aircraft Company, the assignee of the present invention. Itincludes the magnetoresistor 28 connected in series with a fixedresistor R1 in a voltage divider circuit between a positive busv+(typically 5 volts) and ground. A tap take between the magnetoresistor28 and R1 is applied through a second resistor R2 to the inverting inputof an operational amplifier 46, with a ground reference applied to theamplifier's non-inverting input and a feedback resistor R3 connectedbetween its output and inverting input. The magnetoresistor 28 isintended to be positioned within a varying magnetic field, such that itsresistance varies with the strength of the magnetic field at any giventime. For a periodically varying magnetic field, the amplifier's outputVo will thus also vary periodically.

The fixed resistors R1, R2 and R3 are preferably also fabricated fromInSb on the same substrate as magnetoresistor 28, but are not providedwith Hall effect shorting strips. They are accordingly much lessmagnetically sensitive than the magnetoresistor 28 that does have theshorting strips, and for practical purposes may be considered to bemagnetically insensitive. An advantage of fabricating both the fixedresistors and the magnetoresistor from the same material is that theyhave the same temperature coefficients; their resistance values varywith temperature in the same manner. As discussed in U.S. Pat. No.5,038,130, this prevents a variance in the circuit operation that wouldotherwise result from the fixed resistors and magnetoresistor havingdifferent temperature coefficients.

The differential signal of R1 and magnetoresistor 28 is the input to theoperational amplifier 46, while the amplifier's output Vo can be appliedto a conditioning circuit such as the digitizing circuit 48 thatconsists of cascoded FETs 50 and 52. The output Vd of the digitizingcircuit is a TTL (transistor-transistor logic) compatible square wave,with a frequency proportional to the rotational speed of the wheel orother magnetic element sensed by the magnetoresistor 28.

The use of a thin film magnetoresistor greatly reduces the costsassociated with bulk magnetoresistive sensors, while its monolithicintegration with related circuitry enhances the signal-to-noise ratioand allows the output signal to be amplified to a more manageable level.In addition, monolithic integration with GaAs circuits enhances the hightemperature capability of the sensors because of the known hightemperature durability of GaAs integrated circuits. The fabricationprocess described herein is readily adaptable to mass productiontechniques and has many applications, such as for antilock brakesystems, transmission control systems and engine ignition controlsystems in the automotive field; these applications all require amagnetic sensing capability with high sensitivity and high reliability.While a particular embodiment of the invention has been shown anddescribed, numerous variations and alternate embodiments will occur tothose skilled in the art. Accordingly, it is intended that the inventionbe limited only in terms of the appended claims.

We claim:
 1. An integrated magnetoresistive sensor, comprising:asemiconductor substrate, a doped layer in said substrate, amagnetoresistor integrated on said substrate, said magnetoresistorhaving a thickness substantially greater than the thickness of saiddoped layer, and electrical contacts to said doped layer establishing atleast one active electrical device therein, and interconnecting saiddevice with said magnetoresistor.
 2. The integrated magnetoresistivesensor of claim 1, further comprising additional non-magneto resistorsintegrated on said substrate, said non-magneto resistors together withsaid active electrical devices comprising an amplifier for saidmagnetoresistor.
 3. The integrated magnetoresistive sensor of claim 2,said active electrical devices further comprising a digitizing circuitfor the output of said amplifier.
 4. The integrated magnetoresistivesensor of claim 1, wherein said magnetoresistor is formed from InSb andsaid substrate is formed from GaAs.